In ultrasonic diagnosis, the pulsation of the heart or the movement of a fetus can be displayed in real time by the simple operation of bringing an ultrasonic probe into contact with the surface of the body. In addition, this technique is highly safe, and hence allows repetitive examination. Furthermore, this system is smaller in size than other diagnostic apparatuses such as X-ray, CT, and MRI apparatuses. This technique can therefore be said to be a simple diagnostic technique which facilitates examination to be performed by moving the apparatus to the bed side. Ultrasonic diagnostic apparatuses used in this ultrasonic diagnosis vary in type depending on the functions which they have. Some compact apparatuses which have already been developed are small enough to be carried with one hand, and ultrasonic diagnosis is free from the influence of radiation exposure unlike diagnosis using X-rays. Therefore, such ultrasonic diagnostic apparatuses can be used in obstetric treatment, treatment at home, and the like.
For ultrasonic diagnostic diagnosis, recently, an ultrasonic diagnostic apparatus capable of generating and displaying three-dimensional image data has been implemented by using such an ultrasonic diagnostic apparatus. Such an ultrasonic diagnostic apparatus two-dimensionally or three-dimensionally scans ultrasonic waves using an ultrasonic probe having ultrasonic transducers arranged one-dimensionally or two-dimensionally to generate an image corresponding to a two-dimensional region (slice) or three-dimensional region (volume). The apparatus then displays the generated image as a two-dimensional image or three-dimensional image. A doctor observes the displayed ultrasonic image and performs image diagnosis of an affected part.
The image obtained by the ultrasonic diagnostic apparatus contains various kinds of noise and speckle caused by the interference phenomenon of received ultrasonic signals, which often hinder the accurate observation of the position and shape of the boundary of an object tissue. Recently, therefore, various types of processing methods for the reduction of such noise and speckle have been proposed. Typical examples of such methods include the spatial compound method and the persistence method. The spatial compound method is a method of reducing noise and speckle by dividing the reception aperture of a probe into a plurality of patterns and acquiring and adding the amplitudes of a plurality of reception signals corresponding to the respective patterns. The persistence method is a method of reducing noise and speckle by performing weighted addition of the images of a plurality of frames including the current and temporarily consecutive frames and using the result for display.
Consider each image before the processing of reducing noise and speckle. Noise having no correlation with (i.e., independent of) other images is generated. When considering noise which is independently generated in each image, it can be said that averaging operation in the spatial compound method uses the phenomenon that a noise component decreases in proportion to the reciprocal of the square root of the number of times of addition of signal components. Weighted addition in the persistence method aims at the same effect.
The conventional ultrasonic diagnostic apparatus has, for example, the following problem.
That is, methods of reducing noise and speckle, typified by the spatial compound method and the persistence method, uniformly perform addition processing regardless of the positions of the pixels of an image. For this reason, noise is not sufficiently reduced in a portion (pixel), of an image, which is mostly constituted by noise.